Bonding method using photocurable acrylate adhesive containing perester/tautomeric acid adhesion promoter

ABSTRACT

A reactive acrylic adhesive, activated by actinic radiation includes, in addition to a photoinitiator, at least about 1.0 percent by weight of a perester compound or cumene hydroperoxide, and 0.5 percent of a tautomeric organic acid, for enhanced bond strength.

BACKGROUND OF THE INVENTION

Reactive acrylic adhesives are well known in the art, and are widelyutilized due to the numerous advantageous characteristics that theyexhibit. Curing of such adhesives may be activated by heat and/orvarious chemical initiators, and the use of actinic radiation(particularly in the ultraviolet region of the spectrum) has taken onconsiderable importance in recent years. Although such compositions arecommercially available, which cure to strong adhesives, increases inbond strength will usually be considered advantageous, and high bondstrength will in fact be a fundamental criterion for certainapplications.

Accordingly, it is the broad object of the present invention to providea novel reactive acrylic adhesive composition that is activated byradiation and that cures to produce a solid adhesive material thatexhibits high levels of bond strength, particularly to glass.

It is also an object of the invention to provide a novel method forbonding surfaces, and especially surfaces of glass, utilizing such anadhesive composition.

SUMMARY OF THE INVENTION

It has now been found that certain of the foregoing and related objectsof the invention are readily attained by the provision of a liquidcomposition comprising a major proportion of polymerizable acrylatemonomer and a filler providing elastomeric domains, an effective amountof an actinic radiation-responsive photoinitiator, about 1.0 to 5.0percent, based upon the weight of the composition, of a perestercompound, and about 0.5 to 5.0 weight percent of an organic acid that iscapable of cyclic tautomerism, the composition being free fromstabilizers containing sulfur and/or the amine functionality.Preferably, the perester compound will be selected from the groupconsisting of tert-butyl perbenzoate and tert-butyl peroctoate, althoughother peresters, such as di-tert-butyl diperphthalate and2,5-dimethyl-2,5-bis(benzoylperoxy)hexane, may also be used.

Generally, the filler employed will be a reactive acrylated polyurethaneoligomer having a molecular weight of about 400 to 6000; preferably, itwill be a diisocyanate-capped polyether acrylated by reaction withhydroxyethyl acrylate or methacrylate. The acrylate monomer willpreferably be isobornyl acrylate, hydroxyethyl methacrylate,polyethyleneglycol dimethacrylate, trimethylcyclohexyl acrylate, andmixtures of the foregoing. Phenyl ketones, such as1-hydroxycyclohexylphenyl ketone and 2,2-dimethoxy-2-phenylacetophenone, will often be preferred for use as the photoinitiator.

The amounts of the ingredients will preferably be in the ranges of about2.0 to 4.0 percent of the perester compound, about 1.0 to 2.0 percent ofthe tautomeric acid, about 30 to 60 percent of the monomer, about 5 to60 percent of the filler, and about 1.0 to 5.0 percent of thephotoinitiator, all based upon the weight of the composition. Mostdesirably, the composition will additionally include acrylic acid,generally used in an amount of about 2.0 to 6.0 weight percent, althoughas little as 1.0 percent and as much as 10.0 percent may be effectiveand desirable.

Additional objects of the invention are attained by the provision of anadhesive composition containing the ingredients and the amounts thereofset forth above, but wherein cumene hydroperoxide is substituted for theperester compound. Surprising increases in bond strength are alsoachieved with such an adhesive, using about 1.0 to 5.0, and preferablyabout 2.0 to 4.0, weight percent of cumene hydroperoxide.

Other objects of the invention are attained by the provision of abonding method, in accordance with which the adhesive compositionhereinabove described is applied to at least one of the surfaces to bebonded. After effecting contact between the surfaces, the composition isexposed to actinic radiation for a period of time sufficient to effectcuring to an adhesive solid. The method of the invention will often beemployed to greatest advantage for the bonding of parts on which atleast one of the surfaces is glass, and a specific application is forthe bonding of glass to stainless steel, as in the fabrication ofhypodermic syringes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Exemplary of the efficacy of the present invention are the followingExamples:

EXAMPLE ONE

A control adhesive composition ("A") is prepared by admixing thefollowing ingredients in the amounts indicated (in parts by weight):3090 parts of an acrylated polyurethane (toluene diisocyanate-cappedpolyether having a molecular weight of about 400, acrylated by reactionwith hydroxyethyl methacrylate); 3708 parts isobornylacrylate (IBOA),free of sulfur and amine-containing stabilizers; 2163 parts hydroxyethylmethacrylate (HEMA); 618 parts acrylic acid; 93 parts of a silaneadhesion promoter; 25 parts of a hydroquinone stabilizer; and 124 partsof 1-hydroxycyclohexylphenyl ketone, as a photoinitiator. A series ofadhesive compositions (B-F) are produced by adding about 2.0 percent oftert-butyl perbenzoate to composition A, based upon the weight thereof,and the indicated percentage (on the same basis) of maleic acid: zero(composition B); 0.5% (C); 1.0% (D); 1.5% (E) and 2.0% (F).

A quantity of each adhesive composition is applied to the clean, freshsurface of a small piece of cold rolled steel, and a piece of windowglass (with a surface cleaned to free it of any contamination) isbrought into surface contact with the steel piece to spread the adhesivebetween them. The assembly is then exposed through the glass to a beamof UV light (300-400 nanometers) for a period of one minute, followingwhich the parts are tested for bond strength in a tension measuringmachine.

Composition B (perester only added) is found to exhibit a dramaticincrease in bond strength over A. Composition C (perester and 0.5%maleic acid) shows a further substantial improvement, and the strengthof the bond produced by composition D (perester and 1.0% maleic) is verysubstantially higher than that of B (on the order of 50% greater). Theimprovement is maintained in compositions E and F (1.5% and 2.0% levelsof maleic acid, respectively) albeit at a somewhat diminished level.

Utilizing the same perester at various concentrations ranging betweenabout 1.0 to 4.0 weight percent produces comparable results, as doessubstitution of tert-butyl peroctoate for tert-butyl perbenzoate, inequivalent amounts. Excellent results are achieved by using compositionD (described above and cured by UV radiation) to bond stainless steelhypodermic syringe needles to the glass barrels.

EXAMPLE TWO

A composition the same as composition D described in Example One isprepared, substituting however 2.0 weight percent of cumenehydroperoxide for the tert-butyl perbenzoate. When tested in the sameway for effectiveness in adhering steel to glass, an unexpected increasein bond strength, of comparable magnitude, is noted.

The acrylate monomer employed in the instant compositions will generallybe a reaction product of acrylic acid and/or methacrylic acid with oneor more mono- or polybasic, substituted or unsubstituted, alkyl (C₁ toC₁₈), aryl or aralkyl alcohols. Preferred acrylates will often be thosein which the alcohol moiety contains a polar substituent (e.g., anhydroxyl, amine, halogen, cyano, heterocyclic or cyclohexyl group),since cross-linking or other intermolecular bonding is promoted thereby.General disclosure of suitable acrylic ester monomers are provided inthe prior art, such as in U.S. Pat. Nos. 3,218,305 (line 65, column 1through line 47, column 2); 3,425,988 (line 49, column 2 through line21, column 3); 3,651,036 (line 69, column 1 through line 12, column 2);3,658,624 (line 74, column 1 through line 58, column 2); 3,826,756 (line49, column 2 through line 5, column 3); and 3,855,040 (line 20, column 3through line 55, column 4), which portions of the foregoingspecifications are hereby incorporated by reference into thisspecification. It is believed that the choice of appropriate monomerswill be evident to those skilled in the art, and that furthergeneralized description thereof will therefore be unnecessary.

Nevertheless, it might be pointed out that specific acrylates which canadvantageously be employed, alone or in combination, include (inaddition to the HEMA and IBOA used in the foregoing Example) thetetraethyleneglycol, isodecyl and hydroxyethyl esters of acylic acid;the butyl, isodecyl, methyl, tetrahydrofurfuryl, isobornyl, anddicyclopentadienyl esters of methacrylic acid; diacrylates, triacrylatesand tetracrylates (i.e., the polyacrylic and polymethacrylic esters) ofbutyleneglycol, triethyleneglycol, tetraethyleneglycol, polyethyleneglycol, bisphenol A, pentaerythritol (particularly the triacrylateester), trimethylcyclohexyl acrylate, and the like.

Turning now more specifically to the filler, virtually any material thatis soluble (to at least a significant degree) in the monomer fraction,and that serves to toughen, flexibilize and/or strengthen the curedadhesive, may be used. The filler need not be introduced as a rubberysolid; liquids and waxy substances are also entirely suitable. It ismost desirable that the filler be reactive with the monomeric componentsto produce intermolecular bonding, since that will enhance compatibilityand tend to maximize the ultimate properties of the adhesive. In anyevent, it is believed that the effective fillers toughen or otherwisedesirably modify the adhesive by dispersing in the momomer formulationto provide elastomeric domains; generally, they will be rubbery orelastomeric materials, thermoplastic polymers, or macromers.

Although the selection of specific appropriate elastomericdomain-providing fillers will be evident to those skilled in the art, itmight be mentioned that typical suitable materials include vinylpolymers, acrylic polymers, polyester elastomers, glycol polymers,acrylated epoxies, natural and synthetic rubbers, and the like. Moreparticularly, fillers such as VINAC B-7 (polyvinyl acetate sold by AircoChemical Company), polyethyleneglycol 6000, HYCAR CTBN and HYCAR 1022(liquid rubbers sold by the B. F. Goodrich Chemical Company), X-80(polyester oligomer sold by Union Carbide Corporation), and KM-229,KM-288 and KM-323B (arylic elastomers sold by the Rohm & Haas Company)are advantageously used in the formulations of the invention.

In many instances, the urethane polymers and prepolymers will bepreferred, with the latter being especially desirable due to thepotential that they afford for further reaction of their pendantisocyanate groups with a reactive functionality (e.g., an hydroxylgroup) provided by a suitable acrylate monomer. Typical specificurethane elastomers that are advantageously used include the rubber soldby B. F. Goodrich Chemical Company under the trade designation ESTANE5730, and the prepolymers sold respectively by Witco Chemical Companyand by N. L. Industries under the designations CASTOMER 0002 and VORITE.Particularly preferred are such urethanes capped with an acrylicmonomer, such as the polyacrylate esters of organic polyisocyanatesdescribed as "monomers" in the above-mentioned U.S. Pat. No. 3,425,988(the portion of the disclosure thereof in column 2, line 35 throughcolumn 4, line 58 is hereby incorporated hereinto by reference). In viewof all of the foregoing, however, it will be appreciated that theselection of a particular elastomeric domain-providing filler is notcritical, and that the specific material used may vary widely, dependingupon the application for, and the properties desired in, the adhesive.

The amount of filler utilized will depend upon several factors,including again the properties desired in the ultimate product, thenature of the components employed, and the like. Generally, at leastabout 5.0 and less than about 90.0 weight percent will be used; in thepreferred compositions the filler will normally constitute about 20 to50 weight percent.

As will be appreciated, a primary feature of the instant compositionsresides in the use of a perester compound. Although the perestercompounds employed in the instant compositions are widely used for theinitiation of free-radical reactions, they are not employed as catalystshere. Curing of the present adhesives is achieved by activation of thephotoinitiator, using radiation of appropriate wave number; there is noneed to subject the material to heat or chemical activation, as isnecessary, from a practical standpoint, to produce catalytic activity inthe peresters. This is indicated by Example One, wherein addition of theperester compound alone to composition A (to produce composition B)results in a dramatic increase in bond strength, which is not believedto be attributable to enhanced curing of the composition (albeit that,in some instances, the perester may function to increase curing in"shadow" areas, i.e., areas that are not subjected to the full-strengthradiation beam).

The same comments as are made above also apply to the use of cumenehydroperoxide. Although not a perester, it has been found (as indicatedby Example Two) to be highly effective in producing enhanced bondstrength to glass; this is not believed to be true in regard to allperoxygen compounds that are typically used as catalysts.

The tautomeric acid constituent also contributes very substantially tobond strength, achieving rather dramatic increases when employed atoptimal concentrations. Although maleic acid is preferred, other acidscapable of cyclic tautomerism can also be used; such acids includemalic, salicylic, itaconic and phthalic. To avoid possiblemisunderstanding, perhaps it should be pointed out that the term "cyclictautomerism", as used herein, refers to that capability of the acid totheoretically exist in a state in which a bond or pseudo bond is formedbetween the carboxylic moieties of the acid, to produce an isomer inwhich those moieties are in ring-like configuration, in equilibrium witha non-cyclic isomer.

The present combination of ingredients produces particularly desirableresults in regard to the bonding of glass surfaces. Although silanecompounds are conventionally utilized for enhancement of that propertyin products heretofore available, and is desirably incorporated into theinstant compositions as well, surprising increases in bond strength toglass result from the combinations herein provided, when cured byactinic radiation. Since silane is a coupling agent for glass, and theperesters, cumene hydroperoxide and the tautomeric acids are not, theincreases in bond strength would not have been expected.

It has also been found that virtually any compatible photoinitiator maybe employed in the instant compositions, and appropriate compounds willbe evident to those skilled in the art. Nevertheless, it might be notedthat 1-hydroxycyclohexylphenyl ketone, 2,2-dimethoxy-2-phenylacetophenone, diethoxy acetophenone, and2-methyl-1-(methylethiophenyl)-2-(4-morpholinyl)-1-propanone can be usedto good effect for response to actinic radiation in the Uv range, as ispreferred, and camphroquinone and suitably sensitized compositions usingthe above-listed photoinitiators can, for example, be employed invisible-light reactive systems; other appropriate initiators includebenzophenone, benzil, tetramethyldiaminobenzophenone (Michler's ketone),benzoin ethers, alpha,alpha-diethoxyacetophenone and ketocoumarins.Typically, about 1.0 to 5.0 weight percent, and preferably about 2.0 to4.0 percent, of the photoinitiator will be employed, although amounts aslow as 0.1 percent, and as high as 10.0 percent, may be effective anddesirable in some instances.

In addition to the principal components hereinabove described, it willbe evident that other ingredients may also be incorporated into theinstant compositions. For example, acrylic acid, used in the amountsindicated, is especially desirable from the standpoint of maximizingadhesion and curing rate; "inert" fillers, such as wood flour, glassfibers, cotton linters, mica, alumina, silica, and the like, areconventionally used to modify viscosity, improve impact resistance, andfor other purposes, and they may be employed in the instant compositionsif so desired. As indicated above, it is also conventional to includesmall percentages of silane monomers to increase moisture resistance aswell as to enhance the bond strength of the adhesive to glass andsimilar surfaces, and incorporation of such monomers into thecompositions hereof is therefore especially desirable. Other substancessuch as dyes, fire retarders, stabilizers (e.g., the quinones andhydroquinones), thixotropes, thickeners, viscosity reducers,plasticizers, antioxidants, and the like, may additionally beincorporated, although it will be appreciated that such additives willoften be furnished in the principal ingredients, making their separateintroduction unnecessary.

Despite the fact that all of the compositions of the invention exhibitadhesive properties, it will of course be understood that they can beused for other purposes as well. For example, they may serve ascoatings, for applications in which gap-filling is of primaryimportance, etc.

It is important to note that the present compositions excludestabilizers containing sulfur and/or the amine functionality, which arefound to have a most detrimental effect upon the properties of theadhesive. Regard for this restriction is especially important in thoseinstances in which the monomer portion comprises isobornyl acrylate,because phenothiazine (or a like stabilizer) is normally used tostabilize IBOA, absent a special order to the manufacturer.

Thus, it can be seen that the present invention provides a novelreactive acrylic adhesive composition that is activated by actinicradiation and that cures to produce a solid adhesive material havinghigh bond strength, most notably and unexpectedly to glass. Theinvention also provides a novel method for bonding surfaces utilizingsuch an adhesive composition, which method is especially well adaptedfor bonding glass surfaces.

Having thus described the invention, what is claimed is:
 1. In a methodfor bonding surfaces to one another, the steps comprising:(a) providinga liquid composition capable of reaction to an adhesive solid whenexposed to actinic radiation, comprising a major proportion of at leastone polymerizable acrylate monomer and an elastomeric domain-providingfiller, an effective amount of an actinic radiation-responsivephotoinitiator and, based upon the weight of said composition, about 1.0to 5.0 percent of a perester compound having free radical-initiatingcapability and about 0.5 to 5.0 percent of an organic acid capable ofcyclic tautomerism, said composition being free from stabilizerscontaining sulfur or the amine functionality; (b) applying saidcomposition to at least one of the surfaces to be bonded, and effectingcontact of another surface therewith; and (c) exposing said compositionbetween said surfaces to actinic radiation for a period of timesufficient to effect curing of said composition to an adhesive solid. 2.The method of claim 1 wherein, in said composition said perestercompound is selected from the group consisting of tert-butyl perbenzoateand tert-butyl peroctoate, and wherein said organic acid is maleic. 3.The method of claim 1 wherein, in said composition said filler is anacrylated polyurethane oligomer.
 4. The method of claim 3 wherein, insaid composition said oligomer has a molecular weight of about 400 to6000 and is a diisocyanate-capped polyether acrylated by reaction withhydroxyethyl acrylate or hydroxyethyl methacrylate.
 5. The method ofclaim 1 wherein, in said composition said acrylate monomer is a compoundselected from the group consisting of isobornyl acrylate, hydroxyethylmethacrylate, polyethyleneglycol dimethacrylate, trimethylcyclohexylacrylate, and mixtures thereof.
 6. The method of claim 1 wherein, insaid composition said photoinitiator is a compound selected from thegroup consisting of diethoxy acetophenone, 1-hydroxycyclohexylphenylketone, 2-methyl-1-(methylthiophenyl)-2-(4-morpholinyl)-1propanone, and2,2-dimethoxy-2-phenyl acetophenone, and wherein said actinic radiationis in the ultaviolate range.
 7. The method of claim 1 wherein saidcomposition additionally includes about 2.0 to 6.0 percent by weightthereof of acrylic acid.
 8. The method of claim 1 wherein at least oneof said surfaces is glass.
 9. The method of claim 8 wherein a silaneadhesion promoter is additionally included in said composition.
 10. In amethod for bonding surfaces to one another, the steps comprising:(a)providing a liquid composition capable of reaction to an adhesive solidwhen exposed to actinic radiation, comprising a major proportion of atleast one polymerizable acrylate monomer and an elastomericdomain-providing filler, an effective amount of an actinicradiation-responsive photoinitiator and, based upon the weight of saidcomposition, about 1.0 to 5.0 percent of a compound selected from theclass consisting of perester compounds having free radical-initiatingcapability and cumene hydroperoxide, and about 0.5 to 5.0 percent of anorganic acid capable of cyclic tautomerism, said composition being freefrom stabilizers containing sulfur or the amine functionality; (b)applying said composition to at least one of the surfaces to be bonded,and effecting contact of another surface therewith; and (c) exposingsaid composition between said surfaces to actinic radiation for a periodof time sufficient to effect curing of said composition to an adhesivesolid.
 11. In a method for bonding surfaces to one another, the stepscomprising:(a) providing a liquid composition capable of reaction to anadhesive solid when exposed to actinic radiation, comprising a majorproportion of at least one polymerizable acrylate monomer and aelastomeric domain-providing filler, an effective amount of an actinicradiation-responsive photoinitiator and, based upon the weight of saidcomposition, about 1.0 to 5.0 percent of tert-butyl peroctoate and about0.5 to 5.0 percent of an organic acid capable of cyclic tautomerism,said composition being free from stabilizers containing sulfur or theamine functionality; (b) applying said composition to at least one ofthe surfaces to be bonded, and effecting contact of another surfacetherewith; and (c) exposing said composition between said surfaces toactinic radiation for a period of time sufficient to effect curing ofsaid composition to an adhesive solid.
 12. The method of claim 11wherein said organic acid is maleic.